I looked up
cation exchange capacity on the web and found all the uni notes incomprehensible. how does anyone pass? I guess they study chemistry at school for a start.
I also guess, if I understood this, I might figure out why some of my soil has a Ph of 9
This was one of the better ones from Washington State Uni
http://soils.tfrec.wsu.edu/webnutrit...rops/04CEC.htm
Cation-Ex
change Capacity (CEC)
Cation-exchange capacity is defined as the degree to which a soil can adsorb and exchange cations.
Cation-a positively charged ion (NH4 +, K+, Ca2+, Fe2+, etc...)
Anion-a negatively charged ion (NO3 -, PO42-, SO42-, etc...)
Soil particles and organic matter have negative charges on their surfaces. Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles. Once adsorbed, these minerals are not easily lost when the soil is leached by water and they also provide a nutrient reserve available to plant roots.
These minerals can then be replaced or exchanged by other cations (i.e., cation exchange)
Top of page
CEC is highly dependent upon soil texture and organic matter content. In general, the more clay and organic matter in the soil, the higher the CEC. Clay content is important because these small particles have a high ration of surface area to volume. Different types of clays also vary in CEC. Smectites have the highest CEC (80-100 millequivalents 100 g-1), followed by illites (15-40 meq 100 g-1) and kaolinites (3-15 meq 100 g-1).
Examples of CEC values for different soil textures are as follows:
Soil texture
CEC (meq/100g soi)
Sands (light-colored) 3-5
Sands (dark-colored) 10-20
Loams 10-15
Silt loams 15-25
Clay and clay loams 20-50
Organic soils 50-100
In general, the CEC of most soils increases with an increase in soil pH.
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and this academic site
http://www.microsoil.com/CEC.htm
The disadvantages of a low CEC obviously include the limited availability of mineral nutrients to the plant and the soil's inefficient ability to hold applied nutrients. Plants can exhaust a fair amount of energy (that might otherwise have been used for growth, flowering, seed production or root development) scrounging the soil for mineral nutrients. Soluble mineral salts (e.g. Potassium sulfate) applied in large doses to soil with a low CEC cannot be held efficiently because the cation warehouse or reservoir is too small.
and another
The CEC is the abbreviation for the cation exchange capacity of the soil.
Any element with a positive charge is called a cation and refers to the the basic cations, calcium (Ca+2), magnesium (Mg+2), potassium (K+1) and s odium (Na+1) and the acidic cations, hydrogen (H+1) and aluminum (Al+3).
These are from a huge variety of discussion groups all had a slightly differt take on cation exchange capacity. I found them interesting
I hope you do too.
In its simplest form, cation exchange capacity relates to that mineral's ability
to retain nutrients to be absorbed by plant roots. I'm sure a chemist could
provide a much more meaningful definition, but it has to do with positive and
negatively charged ions and their relationship with soil particles.
Pam - gardengal
Topic in rec.gardens
> Cation Exchange Capacity
> The ability of a soil or growth medium to retain nutrients against leaching
> by irrigation water or rainfall is estimated by measuring the cation exchange
> capacity (CEC). Most adsorption sites on growth medium particles are
> negatively charged and attract positively-charged ions. Many nutrients required by
> plants are positively charged and thus are attracted by these
> negatively-charged sites. Sands and other low-surface area materials have low cation exchange
> capacities while organic components have a greater ability to retain
> cations. Pine bark has a cation exchange capacity in the range of 10 to 13
> milliequivalents per 100 cubic centimeters while a CEC of approximately 1 is common
> for builders' sand.
from rec bonsi--
From:
David Hershey - view profile
Date:
Sat, Jan 17 2004 10:38 am
Email:
d ...@excite.com (David Hershey)
Groups:
sci.bio.botany
I would not apply those exact terms to roots. Instead, I would say
roots can excrete acids or bases depending on the environment. Also,
roots have a cation exchange capacity.
Tree roots should be able to excrete acid (hydrogen ions) or bases
(hydroxyl ions or carbonate ions) depending on the ionic composition
of the soil solution.
If most or all of the nitrogen is present as nitrate (NO3-), then
roots excrete hydroxyl ions (OH-). With all nitrogen as NO3-, roots
generally take up more anions than cations so must excrete some OH- to
maintain cation-anion balance. The hydroxyl ions may cause the soil
solution pH to rise. Roots have to have a net cation uptake about
equal to the net anion uptake on a charge basis in order to maintain
electroneutrality.
If a significant amount of nitrogen is present as ammonium (NH4+),
then most roots excrete hydrogen ions (H+) . With a significant amount
of NH4+, roots generally take up more cations than anions so must
excrete some H+ to maintain cation-anion balance. The hydrogen ions
cause the soil solution pH to decline.
In some species of iron-efficient plants, the roots excrete large
quantities of H+ even with all nitrogen as nitrate. This occurs when
the plants become iron deficient. The decline in rootzone pH greatly
increase iron availability. The shrub, Euonymus japonica, responds to
iron deficiency in this way (Hershey and Paul 1983).
In common philodendron (Philodendron scandens ssp. oxycardium), the
roots excrete H+ and the soil solution pH declines even when all
nitrogen is provided as nitrate. This pH decline occurs even when the
plant is not iron deficient (Mattis and Hershey 1992).
The above phenomena have not been studied for too many species.
Roots have a cation exchange capacity because of negative charges on
their cellulose surfaces which are satisfied by cations, such as
calcium (Ca++). Roots have an absolute requirement for calcium and
boron in the external solution to maintain membrane integrity.
from sci bio botany
From:
Bill Robinson - view profile
Date:
Fri, Feb 5 1999 12:00 am
Email:
"Bill Robinson" <rose ...@gte.net>
Groups:
rec.gardens.ecosystems
Nicole wrote:
>Does any one know what buffer capacity is exactly? How
>does it happen? Why? What effect does it have on plants?
>What purpose does it serve? Does it occur in/to all plant
>forms; i.e., shrubs, perennials, etc.? Can you induce it to
>happen?
Righteously good questions! This ought to take about 10
pages and be its own sub-section in the FAQ.
"The Nature and Properties of Soils" by Nyle C. Brady and
Ray R. Weil is a first rate reference and it has a fair amount
of information on soil buffering capacity and the role it plays
in the soil environment. They define buffering capacity as,
"The ability of a soil to resist changes in pH. Commonly
determined by presence of clay, humus, and other colloidal
materials."
"Cation Exchange Capacity" is kind of like that but not really.
Brady and Weil define " Cation Exchange Capacity" as, "The
sum total of exchangeable cations that a soil can absorb.
Sometimes called 'total-exchange capacity', "base exchange
capacity' or 'cation adsorption capacity'. Expressed in
centimoles of charge per kilogram of soil."
Soils with a high soil buffering capacity will also have a high
cation exchange capacity.
Soil pH shapes a lot of the chemical and bio-chemical reactions
that occur in the soil. It all goes back, at bottom line, to the very
complex, inter-reactions involving the soil environment, the soil
organisms, and the living plant roots. Soils that are low in colloidal
material, which on a practical basis means organic, usually don't
perform as well as soils with a high colloidal content but there is
more to it than simple cations. It also involves biology.
Rodale's made the statement many years ago that the way to feed
the garden was to feed the soil organisms. He was right. The issue
is the best way to feed the soil organisms. A soil rich in organic is
good food for the soil organisms and the organic helps to stabilize
the pH which helps in the nutrition in a whole number of ways.
from rec.gardens ecosystems
and from
Craig Bingman
Topic in sci.
aquaria
Do I understand you correctly in that you claim that GAC has a lower
cation exchange capacity than laterite? My interest in using organic matter
to bind ions was sparked by reading 'The Soil-Plant System in Relation to
Inorganic Nutrition' by M. Fried and H. Broeshart from which I quote "the
organic matter also provides a reactive surface which both adsorbs cations
in exchangeable positions formed by COOH and OH groups and also may complex such ions as Fe, Mn, and even Ca and Mg. This adsorbtion and complexing of nutrients can be appreciable --
The cation exchange capacity of humic acid approximates 250 to 400 meq/100g, which is threefold that of the montmorillonite-type clays and 30- to 100-fold that of the kaolinite type."
This indicates that organic matter has a large potential to supply roots
with nutrients. My thoughts about filter carbon were with hopes to take
advantage of the high exchange capacity while eliminating the anaerobic muck
and H2S gas and water over-enrichment that usually results from letting organic matter decay in the substrate.
Upon rereading it seems that they're really making a point about humic acid more than anything, which is presumably not available in filter carbon and probably only available where there is decay.
This agrees with the old advice about adding peat to the substrate.
However, the hope might still be that the COOH and OH groups still exist
in GAC, if it's made from living plants. I have no idea and I hope that
someone else knows if filter carbon is the complicated end result of
processing plants (e.g. coconut) or if it's nearly purely graphite which
utilizes the "hydrophobic" effect to trap organics. (If this is the case then
why do we worry about carbon removing trace elements which exist in ionic form
from the water?)
Here are two possible drawbacks to using carbon. First, it was mentioned
on another recent thread about carbon that some companies process it with
phosphates: enough said. Second, I ran across an excellent review article
in the Journal of Aquatic Plant Management (vol 24, Jan 1986) which cites
research concluding that too much organic carbon in the substrate limits the
the growth of submerged aquatics (Aquat. Bot. 12:157-172, J. Ecol. 71:161-175).
However this was presumed due to high concentrations of organic acids and it
was also noted that "Low level accumulation of organic matter in such sediments
can apparently stimulate growth due presumably to improved ionic exchange
properties and increased sediment nutrient content." So it seems like one
wants to either avoid or use sparsely any organic matter which might decay.
Water movement through the substrate and water changes might circumvent this,
but this danger was the idea behind the filter carbon.
2) So clays may be the way to go. I do have a slight aversion to laterite's
high price in aquarium circles, but what I really dislike about it is that
we (most aquarists, myself included) know little about why it is so much
better at ion exchange than other clay minerals. Yes, I read the thread from
long ago between Jeff Frank, Oleg, George and others on how laterite is a
tropical clay which undergoes weathering over a geologic time scale and has
most Ca and Mg removed and has charged sites which attract nutrient ions, etc.,
etc.. But these are still vague descriptions which fail to tell us why is
is different from other minerals or some stuff I might dig up in my backyard.
Once I know how it works and why it's unique I'll shut up and buy it
.
Here are some more interesting quotes from Fried and Broeshart's book on
the subject of clays: "The secondary minerals [those not present in the
magma] are primarily responsible for many of the phisicochemical properties
of soils that affect plant nutrition. The dominant reactive clay minerals,
including the kaolinites, montmorillonites, and illites, derive their
reactivity not only from their fineness of subdivision and broken exposed
crystal edges but also from isomorphous substitution in the lattice, resulting
in a net negative charge of the clay particle. It is this net negative charge
and the exposed crystal surface that result in the ionic adsorption of
cation, including nutrient cations. -- The dominant reactive clay minerals
found in soils are two-layer non-expanding types and three-layer expanding-
and nonexpanding-types. -- Within the lattice there are always substitutions
in the three layer minerals. -- These substitutions give rise to exchange
properties i.e., a net charge on the lattice resulting in the ability to
adsorb ions. Lattice substitutions are presumably not common in the two layer
type minerals, such as kaolinite, and most of the exchange properties of these
minerals are supposedly the result of the unbalanced structure at broken
edges. -- In many of the 3 layer minerals water can enter between the unit
layers, giving these minerals an expanded structure. -- The concentration of
M(solid) [a nutrient ion in the solid phase] reflects this difference in
capacity to adsorb exchangeable cations. Those soils in which the two layer
clay minerals dominate (e.g. lateritic type soils) typically contain relatively
small amounts of exchangeable cations and have a relatively small capacity to
hold them. Those in which the three layer clay minerals predominate
(e.g. chernozem soils) usually contain large amounts of exchangeable cations
and have a relatively large capacity to hold them." The double dashes above
mean that I left stuff out.
There is a able in the same section labelled "Cation Exchange Capacity
of Clay Minerals." The lowest listed exchange capacity listed is kaolinite
(3-15 meq/100g) which is a two-layer type which they seem to be indicating
is present in the lateritic type soils. The highest on the list is (drum
roll please...) vermiculite (100-150 meq/100g). You can buy vermiculite at
any nursury! This is just an idea but if it is as good as they make it sound
then I'd be willing to dry it out, pulverize it (for maximum surface area) and
see if it can't be made to sink so we can at least use it in the bottom
portion of the substrate. Comments?
3) Fried and Broeshart also talk about uncombined oxides: Oxides of Fe and
presumably Al exist as coatings on the clay particles. Much more is known of
the He oxides owing to the interest of the soil scientist in the nature of
the laterites." Then later "Al and Fe oxides and hydroxides will, depending
on external pH and salt concentration of the ambient soil solution,
disassociate H+ and OH- ions and can therefore adsorb cations and anions
at negative and positive charged spots. The oxide coatings -- provide a
reactive surface capable of retaining certain anions, chief among which
is Phosphate. It is also becoming apparent that the exchange properties of
soil are due to contibutions from oxide coatings." This seems to indicate
that the exchange properties of laterite are not due primarily to the crystal
structure but to a coating of Fe2O3 on it, which would account for its
supposed orange/brown color. This might be easily duplicated by adding an
iron enriching additive with little or no chelating agent to some of the
finer substrate particles, mixing with water, and allowing to dry in the
sun to make a good coating on the particles.
I'm sure any soil scientist could tell us if this is the key property
od laterite, so if you're out ther please post. I haven't seen this stuff
discussed in previous laterite threads. Also, it is frequently mentioned
that laterite adds a good dose of iron for the plants. If the above is true
then this notion is false since the oxide would be unavailable for uptake and
would function instead as a binding site.
There is more interesting stuff in this great book but you can look it up
yourself. I'm not a soil scientist and I found this and other good references
while doing a superficial search on trace element nutrition, so I imagine
there's a wealth of knowledge out there on clay composition and what minerals
work best for nutrition. I think these issues ought to be addressed before
I spend big bucks (on a student budget, not George's) for laterite additives.
Thanks for listening if you're still ther and keep the comments coming
because this subject needs more discussion than lighting or CO2, both of
which are well understood.
Jim Kelly
email jke ...@ucdhep.ucdavis.edu
Well I finally managed to covince my wife I needed some chook poo and horse poo from the local (300m away) farm. ( "No, not more, it smells!")
Us poor peasants have to use ordinary everday poo.
Re: Terra Preta Great web site
) I used to go up to an annual kite retreat at Fort Warden years ago. I have to ask if you were looking South or standing on your head gost when you said 'upper right hand corner of US"? 
semantics is not always just pedantic quibbling. ~ douglas r. hofstadter
